Experimental assessment of power and temperature penetration depth control with a dual frequency ultrasonic system

A novel ultrasound applicator for superficial simultaneous thermoradiotherapy consisting of two parallel-opposed linear arrays and a double-sided scanning reflector was constructed and tested for penetration depth control. In this design the arrays operate at different frequencies (1 and 5 MHz, in this study) and the input power to each array element (five 2 X 2 cm2 elements per array) is computer adjustable. The ultrasonic beams from the arrays are aimed at the scanning reflector which in turn deflects them simultaneously and in parallel toward the treatment volume. Relative intensity distributions generated by the prototype were measured in a degassed water phantom using a thermal technique for a selected reflector position; these showed that the ultrasonic intensity distribution can be controlled in the lateral dimensions by varying the input power level to individual array elements. A fixed-perfused canine kidney phantom was employed to demonstrate experimentally that real time penetration depth control is possible by varying the excitation magnitude of one array (frequency) relative to that of the other. It is concluded that the dual-frequency scanned-reflected ultrasound applicator offers a degree of dynamic three-dimensional control of the power deposition pattern of clinical significance.